Most of electrochemical elements comprise an element unit in which a pair of electrodes are disposed to face each other with a separator therebetween; an electrolyte; and a container for accommodating them. In general, this container is composed of a metal case having an opening, and a sealing body for sealing the opening. Moreover, in order to ensure electrical insulation between the metal case and the sealing body, a gasket is sometimes interposed between them. Further, a sealant is provided between the metal case and the sealing body, or between the gasket and the metal case or the sealing body. Note that the metal case is available in various shapes, such as a cylindrical shape with a bottom, a rectangular shape and a button shape.
The sealant is required to have the functions of preventing the electrolyte from leaking out or evaporating and preventing infiltration of moisture from the outside. In particular, in an electrochemical element using a nonaqueous electrolyte as the electrolyte, since moisture has an adverse effect on the nonaqueous electrolyte and the electrodes, a sealant is used with a view to prevent the infiltration of moisture into the element.
A variety of sealant has been conventionally proposed. Among them, asphalt pitch, coal tar or the like has been most widely used. Moreover, one obtained by mixing the pitch with a mineral oil (Japanese Examined Patent Publication No. 61-36344) and one obtained by mixing the pitch with a silicone rubber (Japanese Laid-Open Patent Publication No. 63-80471) were proposed as the sealants that hardly cause an electrolyte leakage under high temperature/low temperature.
An electrochemical element as described above is directly mounted on a circuit board with a decrease in the size of the element. The major mounting method is automatic soldering by a reflow method. In recent years, in view of various problems in the environmental aspect, there has been a worldwide tendency toward lead-free products. Accordingly, there is urgent necessity to develop parts that can correspond to high-temperature reflow automatic mounting using lead-free solder. However, while the parts to be mounted have been made resistant to high temperature one after another, it is difficult to make electrochemical elements correspond to the high-temperature reflow automatic mounting because they are parts for storing energy, and the realization of high-temperature resistant electrochemical elements as products is much behind as compared to other parts to be mounted. The major reason for this is that the electrochemical element is exposed to a high-temperature condition (under an environment of 250° C. at the peak for around several seconds) in passing through the inside of a reflow furnace, receives extremely high thermal stress and then bursts or causes performance degradation.
In order to solve such problems, the present inventors proposed in the past a sealant made of an elastomer, such as butyl rubber and styrene butadiene rubber, and having both the heat resistant and sealing properties (Japanese Laid-Open Patent Publication No. 2000-138042). This sealant achieved a significant reduction in the rate of occurrence of electrolyte leakage. However, the occurrence of electrolyte leakage was recognized during long-time storage though it was a very few.
As the result of studying in detail the cause of the occurrence of electrolyte leakage, the present inventors found that variations in the applied or coated position of the sealant and the uniformity of a film made of the sealant are the causes of the occurrence of electrolyte leakage. Moreover, since the sealant proposed by the present inventors previously is transparent and colorless, it is extremely difficult to confirm by visual observation or image recognition the applied position and the uniformity of the film in a state where the sealant is applied to the gasket. Therefore, it is considered that elements whose evaluation values about the applied position and the uniformity of the film are out of the specified values can not be removed as defective units on the production process, resulting in the occurrence of electrolyte leakage. In order to prevent the occurrence of such a defect, a prior example (Japanese Laid-Open Patent Publication No. 3-37955) that judges the applied state of the sealant by image data discloses a structure for distinguishing the sealant from a background by adding a white powder or a luminescent material to the sealant as auxiliary means for performing image recognition. However, such a powder and luminescent material have substantially no function as the sealant, and they sometimes cause degradation of the sealing performance.
In order to solve the above-mentioned conventional problems, it is an object of the present invention to provide an electrochemical element having a structure capable of confirming simultaneously the applied position and the uniformity of the film thickness of a sealant applied to the case, sealing plate and gasket of the electrochemical element by visual observation or image recognition, without adversely affecting the characteristics of the sealant.